1. Field of the Invention
The present invention relates to a vacuum brake hose and specifically to a vacuum brake hose for use in a vacuum servo unit for braking.
2. Description of the Related Art
Generally, a vacuum brake hose (or master back hose) used for a vacuum servo unit for braking connects a brake booster and an intake manifold (of an engine) and has functions of absorbing vibrations caused by engine and caused during driving as well as transmitting a negative pressure generated in the intake manifold to the brake booster. The negative pressure generated when the engine introduces air is transmitted to the brake booster so as to be used in a circuit for increasing braking force and also decreasing force required for effecting the brake. Generally, such a brake hose requires extraction resistance (to prevent clogging of a check valve), non-staining properties (by coating), vibration absorbing properties, resistance to negative pressure, heat resistance and the like. With recent upgraded performances of automotive system, conditions (temperature, pressure and the like) of an engine compartment tend to be more severe, so that better performances of such hoses are demanded.
Since oil mist passes through the vacuum brake hose, which requires the above-mentioned performances, oil-resisting rubber such as acrylonitrile-butadiene rubber (NBR) is mostly used as an inner layer thereof, while chloroprene rubber (CR) having relatively good properties of ozone resistance and oil resistance is used as an outer layer thereof. As a hose having such a structure, for example, proposed is a hose having a structure of a tubular inner layer formed by acrylonitrile-butadiene rubber (NBR), a reinforcing fiber layer formed on an outer peripheral surface of the tubular inner layer and an outer rubber layer formed by chloroprene rubber (CR) on an outer peripheral surface of the reinforcing fiber layer, wherein an intermediate adhesive layer formed by chlorosulfonated polyethylene rubber (CSM) is intervened between the tubular inner layer and the reinforcing fiber layer (for example, see Japanese Unexamined Patent Publication No. 08-113769).
Since the hose of the above-mentioned Publication has the intermediate adhesive layer made of CSM, which is a relatively thick film, intervened between the tubular inner layer and the outer layer, there is a problem of cost increase. In the meantime, when the film thickness of the intermediate layer is decreased, interlaminar adhesion between an inner layer and an outer layer becomes insufficient, so that such a hose becomes difficult to sufficiently meet demands such as upgrade performances and prolonged life of the hose product.